Robots.txt vs Rel=Nofollow vs Meta Robots Nofollow : SEO Book.com
http://www.seobook.com/robots-txt-vs-rel-nofollow-vs-meta-robots-nofollow

Aug 6, 2008 - Aaron Wall's chart comparing elements

SEO hard to understand

I was just fixing up our Robots.txt tutorial today, and figured that I should blog this as well. From Eric Enge's interview of Matt Cutts I created the following chart. Please note that Matt did not say they are more likely to ban you for using rel=nofollow, but they have on multiple occasions stated that they treat issues differently if they think it was an accident done by an ignorant person or a malicious attempt to spam their search engine by a known SEO (in language that is more rosy than what I just wrote).